1. Field of the Invention
The invention relates generally to a control unit and control method for a variable valve timing mechanism, a program for implementing the control method, and a recording medium on which the program is recorded. More specifically, the invention relates to a technology for learning the phase of a valve, which is detected by a valve phase detector, in an internal combustion engine in which the phase of at least one of an intake valve and an exhaust valve is variable.
2. Description of the Related Art
A variable valve timing (VVT) mechanism that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing mechanism changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
The range in which the phase is allowed to change may change due to, for example, slack in a chain that connects a crankshaft to the camshaft. Therefore, the phase of the mechanically determined most retarded angle and the phase of the mechanically determined most advanced angle change due to temporal change of an internal combustion engine. In this case, for example, the value output from a cam position sensor, which is stored as an output value corresponding to the phase of the most retarded angle, may differ from the value corresponding to the phase of the actual most retarded angle. Accordingly, it is necessary to periodically learn the phase that is detected by the cam position sensor at the phase of the mechanically determined most retarded angle.
Japanese Patent Application Publication No. 2001-82190 (JP-A-2001-82190) describes a valve timing control apparatus. If a learned value of a deviation from the reference position for a variable valve timing mechanism is cleared, the valve timing control apparatus promptly executes learning for the reference position. The valve timing control apparatus described in JP-A-2001-82190 learns the deviation of the actual valve timing, calculated based on a crank angle and a cam position, from the valve timing corresponding to the reference position for the variable valve timing mechanism, and corrects the actual valve timing. The valve timing control apparatus then controls the variable valve timing mechanism so that the corrected actual valve timing matches a target valve timing that is set based on the engine operating state. The valve timing control apparatus includes a forcibly learning unit that forces the valve timing control apparatus to execute learning using the position corresponding to the target valve timing as the reference position for the variable valve timing mechanism during a set period after an engine is started, when the learned value used to learn the deviation of the actual valve timing from the valve timing corresponding to the reference position for the variable valve timing mechanism is cleared.
With the valve timing control apparatus described in JP-A-2001-82190, when the learned value used to learn the deviation of the actual valve timing, which is calculated based on the crank angle and the cam position, from the valve timing corresponding to the reference position for the variable valve timing mechanism is cleared, learning is forcibly executed using the position corresponding to the target valve timing as the reference position for the variable valve timing mechanism during the set period after the engine is started. Thus, it is possible to promptly learn the deviation of the actual valve timing from the valve timing corresponding to the reference position and reflect the deviation on the valve timing control. Therefore, it is possible to execute the control accurately, thereby eliciting sufficiently high engine output performance.
In order to reduce vibration, that is likely to occur when the engine is being started, the compression ratio at the time of cranking may be reduced by greatly retarding the phase of the intake valve when the engine is being started. In an internal combustion engine in which the phase is controlled in this way, the phase that should be used as the reference phase, for example, the phase of the most retarded angle of the valve is not appropriate as the phase used during an operation of the engine. In this internal combustion engine, if the phase is learned after the internal combustion engine is started, that is, when the internal combustion engine is operating, as described in JP-A-2001-82190, the phase of the valve may be retarded more than necessary. In this case, for example, exhaust emission may deteriorate.
Japanese Patent Application Publication No. 2004-156461 (JP-A-2004-156461) describes a variable valve timing control apparatus that includes a reference position learning unit that learns a reference position for the valve timing before an internal combustion engine is started, and an engine start control prohibition unit that prohibits execution of a start control for the internal combustion engine until learning of the reference position is completed.
With the variable valve timing control apparatus according to JP-A-2004-156461, it is possible to accurately detect the actual valve timing from the beginning of the start of the internal combustion engine, and to reliably control the actual valve timing to a target valve timing that is suitable for starting the internal combustion engine. Therefore, the internal combustion engine is started more smoothly. In addition, it is possible to avoid the situation in which the start control for the internal combustion engine is started before learning of the reference position is completed. Therefore, it is possible to initiate the start control for the internal combustion engine after learning of the reference position is surely completed.
However, as in the case of the variable valve timing control apparatus described in JP-A-2004-156461, if the phase of the valve is learned before the internal combustion engine is started, that is, when a camshaft is stopped, it is hard to change the phase of the valve to the phase that should be learned, because a high resistance is generated when the camshaft is rotated. Therefore, the accuracy of learning may deteriorate. In addition, the vehicle is not allowed to travel until learning of the reference position is completed.